Nitriding process

ABSTRACT

A NITRIDING METHOD FOR CASE HARDENING OF CHROMIUM FERROUS ALLOYS IS DESCRIBED WHICH EMPLOYS A PRETREATMENT OF THE ALLOY TO DEPOSIT NICKEL OR A NICKEL SALT ON THE ALLOY SURFACE, THE PRETREATMENT PERMITS EFFICIENT NITRIDING OF THE ALLOYS WHICH, IN THE ABSENCE OF SUCH PRETREATMENT, RESIST NITRIDING BY THE PRESENCE OF AN OXIDE PROTECTIVE SURFACE.

3,647,572 Patented Mar. 7, 1972 United States Patent Office US. Cl. 14816.6 13 Claims ABSTRACT OF THE DISCLOSURE A nitriding method for case hardening of chromium ferrous alloys is described which employs a pretreatment of the alloy to deposit nickel or a nickel salt on the alloy surface. The pretreatment permits efiicient nitriding of the alloys which, in the absence of such pretreatment, esist nitriding by the presence of an oxide protective surace.

"DISCLOSURE OF THE INVENTION This invention relates to case hardening of ferrous alloys and, in particular, relates to improvements in case hardening by nitriding of chromium ferrous alloys.

Case hardening of ferrous alloys to impart desired hardness and corrosion resistance to the alloy by nitriding of the surface of the alloy is conventionally practiced. In this method, the alloy is exposed to a partially dissociated ammonia atmosphere at a temperature from about 850 to 1100 F. for a time sufficient to permit the nascent nitrogen to diffuse into the alloy surface to a depth of about 0.003 to 0.030 inch, forming nitrides of various alloying elements such as chromium, manganese, alumi num, etc., and thereby imparting a high surface hardness to the alloy case.

Difficulty is experienced when chromium steel alloys are nitrided since these alloys have an oxide film on their surface which strongly resists adsorption of the nitrogen. This difiiculty is experienced with all chromium ferrous alloys to some degree; however, when the chromium content is greater than about 2 percent, the alloy becomes nearly inert to the nitriding treatment.

Various methods have been developed to overcome the resistance of this oxide film. Some methods have attempted to reduce the film; others have incorporated a halogen, hydrohalide or source thereof such as Freon, polyvinyl chloride, etc., in the nitriding chamber.

These prior attempts are not entirely satisfactory. The halogens are corrosive and the equipment and retort used for nitriding must be designed accordingly. The nitriding method is also complicated by the addition of another variable in the process, i.e., control of the concentration of the halide in the nitriding retort.

I have now found that chromium ferrous alloys can be readily nitrided without any need to remove or deactivate the protective oxide film by pretreating the alloy to deposit on its surface nickel or a nickel salt. The amount of nickel so deposited is slight and does not interfere with the specifications of the alloy; however, since nickel alloys are commonly nitrided, nickel is not foreign to the alloy and its presence in the finished product is not usually detectibly different than that of the untreated alloy. That nickel was found active for this pretreatment was particularly surprising in view of Pat. 1,902,676 which teaches that nickel is not suitable as a coating for nitriding steels.

It is believed that the nickel so deposited funuctions to catalyze ammonia decomposition at the metal surface and the resultant nascent hydrogen and nitrogen are sufficiently active to either remove or penetrate the otherwise protective oxide film.

The nickel or nickel salt can be deposited on the alloy surface by any of a plurality of methods. The deposition need not provide a continuous coating of the surface; however, I prefer to apply adequate amounts to provide a thin, nickel-containing film on the surface. The metallic nickel can be deposited by electroplating, vacuum coating, vapor phase reduction or by solution plating. The nickel salts can be deposited on the alloy surface by wetting the surface with a solution of the salt followed by evaporation of the solvent.

The pretreatment can be applied to any chromium ferrous alloy including those which have been or customarily are nitrided. Such alloys have from 1 to about 30 weight percent chromium. Treatment of alloys having greater than about 1 percent and, particularly, alloys having greater than about 2 percent chromium results in the greatest improvement in the subsequent nitriding since the oxide surface of the high chromium alloys greatly inhibits the nitriding in the absence of the pretreatment. The various stainless steels can also be nitrided with use of the pretreatment such as the martensitic stainless steels containing from about 6 to 17 percent chromium, the ferritic stainless steels containing from about 18 to 30 percent chromium and the austenitic stainless steels containing from about 8 to 30 percent chromium and from 3 to about 20 percent nickel as the principal alloying elements. These chromium ferrous alloys can contain other alloying elements in amounts from about 0.1 to about 20 percent, generally from about 1 to 10 percent.

Typical of such elements of which one or more can be incorporated in the chromium ferrous alloys include: carbon, sulfur, phosphorus, silicon, manganese, aluminum, tungsten, columbium, molybdenum, vanadium, copper, titanium, nickel, zinc, tin, etc.

Some alloys have been developed expressly for nitriding to provide a maximum hardness in the case. These generally contain from about 0.5 to about 3.0 percent chromium together with minor amounts of other alloying elements such as aluminum, molybdenum, manganese and silicon. Typical of these are the Nitroalloys having the following approximate composition.

Weight percent Carbon 0.2-0.4

Aluminum 0.85-1.2

Chromium 0.9-1.8 Molybdenum 0.15-1.0 Manganese 0.4-1 .1 Silicon 0.2-0.4

The alloys are usually heat treated prior to nitriding and preferably, this heat treatment also precedes the pretreatment of this invention. The heat treatment comprises heating the metal to a temperature exceeding that to be experienced in the nitriding step followed by a quenching or tempering step. This heat treatment is applied to impart desired mechanical properties to the core of the metal and to prevent distortion of the metal that may otherwise occur from the heat treatment which accompanies the nitriding step.

While alloy surfaces which are passivated by the presence of a protective oxide film can be successively nitrided by the use of our pretreatment without any removal of the oxides, some surface treatment can be used to improve the nature of the nitriding step. This surface treatment is primarily to remove any surface scale, grease and other surface contaminates. While some of the surface treatments may also remove the oxide surface, there is no necessity to nitride the alloy before the oxide surface is reestablished.

Preferably, the alloy surface is degreased by vapor phase degreasing wherein the metal object is suspended in hot vapors of a solvent such as a chlorinated hydrocarbon, e.g., trichloroethylene, chloroethylene, etc. The vapors condense on the surface and wash the surface of any oil soluble contaminates. This step is commonly performed by suspending the metal part above a vessel of the solvent which is maintained under refluxing conditions. The metal object can also be subjected to conventional vapor honing treatment.

The alloy surface can then be cleaned by treatment with an aqueous solution of an alkaline cleaning compound such as an'alkali metal silicate, hydroxide and/ or phosphate, e.g., sodium silicate, potassium hydroxide, trisodium orthophosphate, etc. This treatment removes any grease or oil that may remain from the vapor honing treatment. The treatment can be at a temperature from 70 to 200 F. and can be performed by immersing the part in a bath of the solution or by Washing the surface with a spray of the solution. The part can then be rinsed with water to remove all traces of the alkaline solution.

Any or all of the aforementioned preparative treatments can be practiced or can be ignored as desired when using the nitriding pretreatment of my invention. Although the vapor honing treatment can remove some of the protective oxide film from the alloy surface, there is no necessity for the metal part to be immediately nitrided before this film restores itself since my pretreatment activates even the oxide coated alloy surfaces.

The nickel can be deposited on the alloy surface by electroplating from an aqueous solution of a nickel salt such as a buffered solution of nickel chloride or sulfate, or other soluble nickel salts described hereafter. In this procedure, conventional electroplating techniques can be used wherein a direct current voltage is applied to the metal part to make it cathodic with regard to the solution using current densities of from to about 50 amperes per square foot and potentials from about 5 to about volts.

The nickel can also be vacuum coated on the surface by placing the metal part together with a supply of nickel in a vacuum chamber, evacuating the chamber to less than about 10 microns mercury pressure and heating the nickel to about 1500 to 2500 F. by a resistance heater to volati-, lize the nickel and cause the nickel to condense on the cold alloy surface. Alternatively, a negative voltage of about 200 to 1000 volts can be applied to the nickel to drive atoms of nickel into the evacuated chamber to condense on the alloy surface.

The nickel film can also be formed on the alloy surface by chemical plating wherein the surface of the metal part is wetted to the point of run off with an aqueous solution of a soluble nickel salt such as salts of inorganic acids such as nickel halides, e.g., nickel chloride, bromide, fluoride or iodide; nickel sulfate; nickel nitrate as well as the water soluble C to about C carboxylates, e.g., nickel,

formate, acetate, propionate, bu'tyrate, isobutyrate, valerate, etc.

The concentration of the salt in the aqueous solution is not critical and the salt can be present up to its solubility limit, e.g., up to about 30 weight percent, calculated as nickel. Preferably, the concentration is from about 1 to 20;'most preferably, from about 3 to 5 weight percent.

The aqueous solution of the nickel salt is preferably maintained under acidic conditions, i.e., at a pH less than about 6.5; preferably less than about 5.0 and, most preferably, less than about 4.0. The acidity can be adjusted by controlled additions of various acids such as the aforementioned inorganic acids, e.g., the hydrohalic acids, nitric, sulfuric, or the C to C alkanoic acids, e.g., formic, acetic, valeric, etc. The commonly used buffering agents such as sodium acetate, carbonate, etc. can also be supplied to control the pH in the aforementioned ranges.

The alloy surface can be contacted with the solution by any suitable method that will wet the surface to the point of runoff. Commonly, the alloy can be simply immersed by dipping into a vat of the solution, however, the solution can also be sprayed or brushed onto the surface. The time of contact between the solution and the surface is not 4 critical and can be from'a fewsecondsto several hours; preferably from about 1 to about 20 minutes.

The temperature of the surface and the solution during the contacting can be from about 35 to about 210 F., preferably from about 50 to about F. and, most preferably, ambient.

Nickels salts can also be deposited on the alloy surface by wetting the surface with a solution of the salt in a volatile solvent such as water or a low boiling organic solvent such as an alkanol, ester or ketone having a boiling point less than about 200 F. and, preferably, less than about F. Examples of these are: methyl formate, ethyl formate, acetone, methyl acetate, methanol, ethyl acetate, isopropanol, ethyl propionate, ethanol, propyl acetate, butanol, diethyl ketone, methylethyl ketone, etc. The solution can contain up to saturation of the nickel salt, generally from about 0.1 to 20 weight percent of the nickel salt. I

The nitriding treatment of the pretreated-alloy is conventional and employs temperatures below the austenite transformation temperature, e.g., from about 850 to 1100 F., typically from about 900 to 1025 F. The alloy is exposed to partially dissociated ammonia vapors at the aforementioned temperatures in a closed retort that is commonly maintained at atmospheric pressure, although pressures from 1 to about 100 atmospheres can be used. The ammonia is maintained at about :15 to 30 percent dissociated, generally by control of the ammonia circulation rate through the retort. The ammonia is generally supplied at a how rate of from 15 to about 35 standard cubic feet per hour per 100 square feet of alloy surface. I

Steels that are alloyed for nitridingfrequently develop a light etching constituent next to the surface of the coating which is very rich in nitrogen when treated in the aforedescribed manner. These steels can be further treated by a second nitriding step at a slightly higher temperature, e.g., from about 1025 to 1050 F. with-an increased ammonia dissociation from about 75 to percent dissociation. This second step'reduces the nitrogen content of the outer layer by diffusion of the nitrogen more deeply into the alloy case;

The time of the nitriding can be from about 3 to 60 hours, commonly about 5 to 25 hours. The particular combination of time and temperature used .is chosen to obtain a case depth of ,from about 0.001 to 0.300 inch,

preferably from about 0.002 to 0.015 inch. With the.

aforementioned alloys and particularly the nitriding alloys, case hardnesses of from about 700 to about 1150 Knoop can be achieved, These values of case hardnesses correspond to about from 64 to 75 Rockwell C equivalent.

Stainless steels which have been nitridedca'n'be passivated to impart corrosion resistance ,by contacting the surface for a period of about 5 to 45 minutes, preferably.

from 10 to 20 minutes, with an aqueous solution of ni r c acid and a soluble dhromate. A suitable solution for this purpose is an aqueous solution of 18 to25 percentby volume of 70 weight percentnitric acid and 2 weight, percent sodium dichromate. This h passivation' treatment can be performed at a temperature from about 50. to 200 F.; preferably at ambient temperature.

The following examples will illustrate the presently contemplated best mode of practice of the invention and will also serve to illustrate the results obtainable thereby:

EXAM PIJESl A metal part is nitrided according to the invention. The metal is of the following composition:

The surface of the cleaned metal part is wetted with a methanol solution containing about 6 weight percent nickel chloride hexahydrate by dipping the part into a bath of the solution. The part is removed from the bath and the methanol is permitted to evaporate so as to deposit the nickel salt on the alloy surface.

The metal part is then placed in a gas tight retort which is closed and purged with nitrogen to remove all oxygen and then filled with ammonia at atmospheric pressure. The retort is then heated to 950 F. and maintained at that temperature for 24 hours while ammonia vapor is circulated through the retort at atmospheric pressure. The retort is permitted to cool to ambient temperature after the 24 hour nitriding period while maintaining the ammonia flow. The metal part is removed from the retort and is lightly brushed to remove any residual metal salts. The metal surface is found to have a surface hardness of 1100 Knoop, equivalent to about 71 Rockwell C.

When the procedure is repeated, but with deletion of the surface cleaning steps, substantially the same results are obtained.

When the procedure is repeated with the substitution of pretreating the metal part by dipping it into an aqueous solution of weight percent nickel chloride in dilute hydrochloric acid having a pH of about 4, substantially the same results are obtained.

When the procedure is repeated with the substitution of electroplating of nickel on the alloy surface by placing the part in an aqueous solution of nickel chloride maintained at 140 F. and applying a negative direct current voltage thereto of about 6 volts for a period of one minute, substantially the same results are obtained.

When the nitriding is attempted without any prior treatment with nickel, the alloy surface resists nitriding.

We claim:

1. In the method for case hardening a metal object formed of a chromium ferrous nitridable alloy by exposure of the object to an atmosphere consisting of partially dissociated ammonia for a time and at a temperature sufficient to nitride the case of said object to a depth of from 01001 to 0.300 inch, the improved method of nitriding objects formed of alloys having amounts of chromium from :1 to about 30 percent and sufiicient to impart thereto an oxide surface that resists nitriding which comprises contacting said object having said resistant oxide surface with said partially dissociated ammonia atmosphere in the presence of a deposit of a nickel salt on the surface of said object.

2. The method of claim 1 wherein nickel halide is deposited on said surface.

solution of a soluble nickel salt in a solvent having a boiling point less than about 200 F. and permitting said solvent to evaporate.

7. The method of claim 1 wherein said deposit is pro- :vided by contacting said surface of said object with an aqueous solution of a water soluble nickel salt having a pH less than about 6.5.

8. The method of claim 1 wherein said chromium ferrous alloy contains from 0.1 to about 20 percent of a nitriding element selected from the class of aluminum, molybdenum, manganese and silicon.

9. The method of claim 1 wherein said object is washed with a chlorinated hydrocarbon solvent to remove oil soluble contaminates before said nitriding.

10. The method of claim 1 wherein said object is washed with an aqueous solution of an alkali metal silicate, hydroxide or phosphate before said nitriding.

11. The method of claim 7 wherein said pH of said solution is less than 5.0

12. The method of claim 7 wherein said pH of said solution is less than 4.0.

13. The method of claim 6 wherein said solvent is methanol.

References Cited UNITED STATES PATENTS 1,902,676 3/ 1933 Sutton l4816.6 1,929,252 10/ 1933 Morris 1481 6.6 1,958,575 5/ 1934 Hengstenberg 148-l16.6 3,519,257 7/1970 Winter et al. 148-1 6.5 1,736,919 11/1929 Kinzel !14-8-I16.6 1,748,378 2/1930 Armstrong 148--1'6.6

OTHER REFERENCES Metals Handbook, vol. 2, 8th Ed., 1964, pp. 317 and 334.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

